Rear underbody structure of an automobile

Abstract

It provides a rear underbody structure for vehicles with a reduced number of parts, superior safety performance, and optimized total weight. [Solution] A rear underbody structure (2) for an automobile, comprising first and second side members (4) and at least one cross member (5) connecting the first and second side members (4), wherein the rear underbody structure (2) is made by stamping a single tailored blank having at least two sub-blanks.

Description

Technical Field

[0001] The present invention relates to a rear underfloor structure of an automobile.

Background Art

[0002] Automobile manufacturers are facing increasingly stringent requirements: improving the passive safety of vehicles, reducing vehicle weight to minimize greenhouse gas emissions in the case of internal combustion engine vehicles, increasing the driving range of vehicles in the case of electric vehicles, while keeping production costs low and maintaining high production speeds.

[0003] The rear underfloor structure of an automobile is an important structural element of a vehicle and contributes to the safety of occupants in the event of a rear collision. It protects the gas tank, which is often located under the occupant seat in the case of an internal combustion engine vehicle. It protects the rear electric engine in the case of electric or hybrid vehicles.

[0004] The rear underfloor structure is composed of a large number of individual parts and accounts for a considerable mass of the vehicle body. It also involves an expensive manufacturing process including multiple forming operations and assembly steps to obtain the finished structure.

Summary of the Invention

Problem to be Solved by the Invention

[0005] It is an object of the present invention to address the combined problems of safety, weight reduction and high productivity by providing a rear underfloor structure having a reduced number of parts, excellent safety performance and an optimized total weight.

Means for Solving the Problem

[0006] To this end, the present invention provides: -The present invention relates to a rear underbody structure for an automobile, comprising first and second side members and at least one cross member connecting the first and second side members, wherein the rear underbody structure is made by stamping a single tailored blank having at least two sub-blanks.

[0007] According to other optional features of the rear underbody structure according to the present invention, which are considered individually or in any possible technical combination, - The rear underbody structure is made using hot stamping.

[0008] - The rear underbody structure includes at least two cross members.

[0009] - The rear underbody structure has two side members, and each side member is, -A horizontal wall and an inner wall connected by an inner range, - comprising at least one crossmember mounting zone corresponding to a portion of a side member to which a corresponding crossmember is attached, wherein the inner range extends along at least a portion of the crossmember mounting zone.

[0010] - The tailored blank used to manufacture the rear underbody structure comprises at least one sub-blank coated with an aluminum-based metal coating.

[0011] - The tailored blank used to manufacture the rear underbody structure comprises at least one subblank coated with an aluminum-based metal coating, the subblank containing 2.0–24.0 wt% zinc, 1.1–12.0 wt% silicon, optionally 0–8.0 wt% magnesium, and optionally additional elements selected from Pb, Ni, Zr, or Hf, with each additional element present in a content of less than 0.3 wt%, the remainder being aluminum and optionally unavoidable impurities.

[0012] - The tailored blank used to manufacture the rear underbody structure comprises at least one sub-blank having an emissivity-increasing upper layer on at least one side.

[0013] - The tailored blank used to manufacture the rear underbody structure comprises at least one subblank, the subblank comprising at least one subblank made of press-hardened steel having an extreme tensile strength after hot stamping exceeding 1800 MPa.

[0014] - A tailored blank used to manufacture the rear underbody structure comprises at least one subblank, the subblank being made of press-hardened steel having a hot-formed yield strength between 700 and 950 MPa, a hot-formed ultimate tensile strength between 950 and 1200 MPa, and a hot-formed bending angle greater than 75°.

[0015] - The tailored blank used to manufacture the rear underbody structure comprises at least one sub-blank having at least one metal patch.

[0016] - The tailored blank used to manufacture the rear underbody structure comprises at least one subblank having at least one metal patch including an emissivity-increasing upper layer.

[0017] - A tailored blank used to manufacture the rear underbody structure comprises at least one sub-blank having at least one weld seam reinforcement patch, the weld seam reinforcement patch being applied to the area having a weld seam.

[0018] - The tailored blank used to manufacture the rear underbody structure comprises at least one sub-blank having at least one weld seam reinforcement patch, the at least one weld seam reinforcement patch itself having an emissivity increasing upper layer.

[0019] - The thickness of the interdiffusion layer in the aluminum-based metal coating region of the rear underfloor structure is between 3 microns and 15 microns.

[0020] - The thickness of the interdiffusion layer in the aluminum-based metal coating region of the rear underfloor structure is between 3 microns and 10 microns.

[0021] The present invention further relates to a method for manufacturing the above-mentioned rear underfloor structure, and an automobile provided with the above-mentioned rear underfloor structure.

[0022] Other aspects and advantages of the present invention will become apparent upon reading the following description, given by way of example and made with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] [Figure 1] Fig. 1 is an overall perspective view of a vehicle according to the present invention. [Figure 2] Fig. 2 is a perspective view of a conventional rear underfloor structure according to the known prior art in an assembled form (upper part) and an exploded view (lower part). [Figure 3] Fig. 3 is a top view of an embodiment of the rear underfloor structure according to the present invention. [Figure 4] Fig. 4 is a top view of an embodiment of the rear underfloor structure according to the present invention. [Figure 5] Fig. 5 is a perspective view of an embodiment of the rear underfloor structure according to the present invention. [Figure 6] Fig. 6 is a perspective view of an embodiment of the rear underfloor structure according to the present invention, seen from the A-A cross-section shown in Fig. 3. [Figure 7] Fig. 7 is a perspective view of an embodiment of the rear underfloor structure according to the present invention, in the box region labeled B in Fig. 3. [Figure 8] Fig. 8 is a top view of a tailored blank used for manufacturing the rear underfloor structure according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] In the following explanation, the terms “upper,” “lower,” “front,” “rear,” “transverse,” and “longitudinal” are defined according to the normal orientation of the vehicle on which they are installed. More specifically, the terms “upper,” “lower,” “up,” “down,” “bottom,” and “top” are defined according to the vehicle’s elevation angle; the terms “front,” “rear,” “forward,” “backward,” and “longitudinal” are defined according to the vehicle’s longitudinal direction; and the term “transverse” is defined according to the vehicle’s width. The terms “inner” and “outer” are defined according to the vehicle’s interior and exterior, with “inner” parts, sections, zones, etc. being closer to the interior of the vehicle than the corresponding “outer” parts, sections, zones, etc. The term "height" refers to the distance between two points, lines, surfaces, or volumes measured horizontally.

[0025] For better understanding, in all diagrams showing the rear underbody structure, the rear and front directions of the vehicle are indicated using arrows labeled "R" and "F," respectively.

[0026] A steel blank refers to a flat sheet of steel cut into any shape suitable for its use. The blank has a top and bottom surface, also called the top and bottom or top and bottom. The distance between these surfaces is specified as the thickness of the blank. The thickness can be measured, for example, using a micrometer with its spindle and anvil positioned on the top and bottom surfaces. Similarly, the thickness can also be measured on a molded part.

[0027] Tailored blanks are manufactured by assembling multiple blanks of steel, known as subblanks, together, for example by laser welding, to optimize the performance of the part in its different areas, reduce the overall part weight, and lower the overall part cost. The subblanks that form a tailored blank can be assembled with or without overlap, for example, by laser butt welding (no overlap) or by spot welding to each other (with overlap).

[0028] Hot stamping is a forming technique that involves heating a blank to a temperature at which the steel's microstructure transforms at least partially into austenite, stamping the blank to form a blank at a high temperature, and rapidly cooling the formed part to obtain a microstructure with very high strength. Hot stamping makes it possible to obtain parts with complex shapes and very high strength without springback. To bring about the aforementioned advantages of hot stamping, the material used is known as a press-harden material, which has a chemical composition that allows it to form the desired hardened microstructure when subjected to the hot stamping process described above. It should be understood that the heat treatment to which the part is subjected includes not only the aforementioned thermal cycle of the hot stamping process itself, but also the subsequent paint baking process performed after the part has been painted to bake the paint. The mechanical properties of the hot-stamped parts below were measured after the paint baking process, assuming that the paint baking process was actually performed.

[0029] Yield strength, ultimate tensile strength, and uniform elongation and total elongation are measured in accordance with ISO standard ISO 6892-1, published in October 2009.

[0030] The bending angle is measured according to the VDA-238 bending standard. For the same material, the bending angle depends on the thickness. For simplicity, the bending angle values ​​in this invention refer to a thickness of 1.5 mm. If the thickness is different from 1.5 mm, the bending angle value must be adjusted by the following calculation, where α1,5 α is the bending angle at 1.5 mm, t is the thickness, and α t is the bending angle relative to the thickness t:

[0031]

number

[0032] The bending angle of a component is a method of measuring its ability to resist deformation without forming cracks.

[0033] Emissivity is the relative power of a surface to emit heat through radiation. It represents the ratio of the radiant energy emitted by a surface to the radiant energy emitted by a blackbody at the same temperature, and is a value that falls between 0 and 1. The higher the emissivity of the blank's surface, the more heat the blank absorbs through radiation, and therefore, the easier it is to heat the blank using a radiant furnace.

[0034] A rear underbody structure 2 for automobile 1 is described with reference to Figure 1. Automobile 1 can be any type of passenger car having at least a front and rear set of doors, such as a compact, sedan, or sports utility vehicle. The rear underbody structure described is essentially the same regardless of the vehicle category. Furthermore, the powertrain of the automobile may be an internal combustion engine, an electric motor, a fuel cell, or any type of hybrid system.

[0035] The rear underbody structure 2 extends longitudinally from the rear of the vehicle to below the floor panel of the passenger compartment. The rear underbody structure comprises at least first and second side members 4 located on both sides of the vehicle, and at least one cross member 5 connecting the first and second side members 4 laterally.

[0036] The following is a general description of a typical side member 4, and it should be understood that the following description of the side member 4 does not limit the scope of the present invention and can be applied to any type of design of the side member 4.

[0037] - The side member extends substantially longitudinally at the same height as the rear bumper assembly 11 and has a rear end that is attached to the rear bumper assembly 11 at its rear end. - The front section extends substantially longitudinally at a lower height than the rear section and is attached to the vehicle lateral reinforcement structure 12. - The transition zone comprises at least upper and lower bends connecting the front and rear sections. The presence of these upper and lower bends allows for a difference in height levels between the front and lower sections.

[0038] Referring to Figure 5, the side member 4 has a substantially U-shape comprising a horizontal wall 41 extending substantially in a horizontal plane and an inner wall 42 and an outer wall 43 extending substantially in a vertical plane. The inner wall 42 and the outer wall 43 are connected to the horizontal wall 41 by an inner range portion 45 and an outer range portion 46, respectively. A “range portion” means a part of the component that is substantially curved and allows transitions between different planar directions within the component. In this case, the inner range portion 45 allows transitions between the substantially horizontal orientation of the horizontal wall 41 and the substantially vertical orientation of the inner wall 42. Similarly, the outer range portion 46 allows transitions between the substantially horizontal orientation of the horizontal wall 41 and the substantially vertical orientation of the outer wall 43.

[0039] In certain embodiments, the side member 4 also includes a flange 47 extending along the upper parts of the inner wall 42 and outer wall 43 in a substantially vertical plane. Advantageously, the flange allows the side member 4 to be assembled with the rest of the vehicle structure, for example, by spot welding or laser welding along the flange 47.

[0040] The rear underbody structure 2 further comprises at least one cross member 5 connecting the first and second side members 4. The cross member 5 extends substantially along the transverse direction. Similar to the side members 4, the cross member 5 comprises a horizontal wall 51 extending in a substantially horizontal plane and a front wall 52 and a rear wall 53 extending in a substantially vertical plane. Referring to Figure 3, the rear underbody structure 2 may have one cross member 5. Referring to Figure 4, the rear underbody structure 2 may have two cross members 5. Other embodiments of the present invention are also possible in which the rear underbody structure 2 comprises three or more cross members.

[0041] Referring to Figures 5 and 7, each side member 4 of the present invention includes at least one cross member mounting zone 20 corresponding to the portion of the side member 4 to which the corresponding cross member 5 is attached. The cross member mounting zones 20 are demarcated by dashed lines 21 in Figures 5 and 7. It will be easily understood that there are the same number of cross member mounting zones 20 for each side member 4 as there are cross members 5.

[0042] The latest technology rear underbody structure shown in Figure 2 is made up of several separate parts that are assembled together after being formed. In the latest technology example shown in Figure 2, which illustrates a structure with only one cross member, there are three separate parts.

[0043] In contrast to the latest technologies, the rear underbody structure 2 of the present invention is manufactured by stamping a single tailored blank comprising at least two sub-blanks. This allows for optimization of the performance of the part in its different areas, reduces the overall part weight, and lowers the overall part cost and manufacturing complexity. An example of a tailored blank 26 used to manufacture the rear underbody structure 2 according to the present invention is shown in Figure 8. The weld seam 25 is embodied by the lines in Figure 8. This is an example of a tailored blank configuration. The location of the weld seam, as well as the choice of grade and thickness distribution, is determined by the specific requirements of the vehicle into which the part is integrated. For example, the rear of the side member 4 is made of a material that can absorb collision energy by deforming, while the front of the side member 4 is made of an impermeable material that does not deform during a collision to protect occupants sitting in the rear of the vehicle.

[0044] Among other features, the fact that the parts are punched out from a single tailored blank means that they must conform to the shape of the inner wall 42 of at least one crossmember mounting zone 20. In certain embodiments, the inner wall 42 is not present in the crossmember mounting zone 20. In certain embodiments, the inner wall 42 does not have the same height inside the crossmember mounting zone 20 and outside the crossmember mounting zone 20.

[0045] In certain embodiments, the rear underbody structure 2 is manufactured by hot stamping. Advantageously, this allows for very high mechanical strength despite the complex part geometry, and also allows for very good geometric tolerances of the finished part due to the absence of springback. This last point is significant and particularly important in the case of the rear underbody structure 2, which would suffer high geometric inaccuracies and overall shape defects such as twisting or warping if a considerable amount of springback occurred.

[0046] In certain embodiments, at least one subblank of the blank 26 used to manufacture the rear underbody structure 2 is made of a material such that the yield strength of the corresponding region of the rear underbody structure 2 after press curing is between 700 and 950 MPa, the tensile strength is between 950 and 1200 MPa, and the bending angle is greater than 75°.

[0047] In a particular embodiment, at least one subblank of the blank 26 used to manufacture the rear underbody structure 2 contains a composition comprising, by weight %, 0.06% ≤ C ≤ 0.1%, 1% ≤ Mn ≤ 2%, Si ≤ 0.5%, AI ≤ 0.1%, 0.02% ≤ Cr ≤ 0.1%, 0.02% ≤ Nb ≤ 0.1%, 0.0003% ≤ B ≤ 0.01%, N ≤ 0.01%, S ≤ 0.003%, P ≤ 0.020%, and less than 0.1% Cu, Ni, and Mo, with the remainder being iron and unavoidable impurities resulting from processing. Within this composition range, the yield strength of the corresponding region of the rear underbody structure 2 after press hardening falls between 700 and 950 MPa, the tensile strength falls between 950 and 1200 MPa, and the bending angle exceeds 75°. For example, this subblank is made of Ductibor(R) 1000.

[0048] In a particular embodiment, at least one subblank of the blank 26 used to manufacture the rear underbody structure 2 has an ultimate tensile strength in the region corresponding to the subblank after press hardening that is between 1300 and 1650 MPa and a yield strength in the region between 950 and 1250 MPa.

[0049] In a particular embodiment, at least one subblank of a blank 26 used to manufacture the rear underbody structure 2 has, in the region corresponding to the subblank after press hardening, an ultimate tensile strength between 1300 and 1650 MPa, a yield strength between 950 and 1250 MPa, and a bending angle greater than 75°.

[0050] In a particular embodiment, at least one subblank of the blank 26 used to manufacture the rear underbody structure 2 contains, for example, 0.20%≦C≦0.25%, 1.1%≦Mn≦1.4%, 0.15%≦Si≦0.35%, ≦Cr≦0.30%, 0.020%≦Ti≦0.060%, 0.020%≦Al≦0.060%, S≦0.005%, P≦0.025%, 0.002%≦B≦0.004% by weight, with the remainder being iron and unavoidable impurities resulting from processing. Within this composition range, the ultimate tensile strength of the part in the region corresponding to the subblank after press hardening falls between 1300 and 1650 MPa, and the yield strength falls between 950 and 1250 MPa. For example, the subblank is made of Usibor(R)1500.

[0051] In certain embodiments, at least one subblank of the blank 26 used to manufacture the rear underbody structure 2 is made of a material such that the tensile strength of the corresponding area of ​​the rear underbody structure 2 after press curing is greater than 1800 MPa.

[0052] In a particular embodiment, at least one subblank of the blank 26 used to manufacture the rear underbody structure 2 contains, in weight percent, 0.24%≦C≦0.38%, 0.40%≦Mn≦3%, 0.10%≦Si≦0.70%, 0.015%≦Al≦0.070%, Cr≦2%, 0.25%≦Ni≦2%, 0.015%≦Ti≦0.10%, Nb≦0.060%, 0.0005%≦B≦0.0040%, 0.003%≦N≦0.010%, S≦0.005%, P≦0.025%, with the remainder being iron and unavoidable impurities resulting from processing. Within this composition range, the tensile strength of the corresponding region of the rear underbody structure 2 after press hardening is higher than 1800 MPa. For example, this subblank is made of Usibor(R)2000.

[0053] In certain embodiments, the blank 26 used to manufacture the rear underbody structure 2 comprises at least one subblank coated with an aluminum-based metal coating. "Aluminum-based" means a coating containing at least 50% aluminum by weight. For example, the metal coating is an aluminum-based coating containing 8-12% Si by weight. The metal coating is applied, for example, by immersing the substrate in a molten metal bath. Advantageously, the application of the aluminum-based metal coating avoids the formation of surface scale during the heating step of the hot stamping process, thereby enabling the production of parts by hot stamping without subsequent sandblasting. Furthermore, the aluminum-based coating also provides corrosion protection to the parts during use in a vehicle.

[0054] In certain embodiments, the blank 26 used to manufacture the rear underbody structure 2 comprises at least one subblank coated with an aluminum-based metal coating, the aluminum-based metal coating containing 2.0–24.0 wt% zinc, 1.1–12.0 wt% silicon, optionally 0–8.0 wt% magnesium, and optionally additional elements selected from Pb, Ni, Zr, or Hf, with each additional element present in a content less than 0.3 wt%, the remainder being aluminum and optionally unavoidable impurities. Advantageously, this type of metal coating provides very good corrosion resistance on the part, as well as good surface characteristics after hot stamping.

[0055] Laser welding can be used to manufacture the aforementioned tailored blanks having an aluminum-based coating on at least one of the subblanks. It is possible to use subblanks with pre-prepared edges to be welded by ablating a portion of the metal coating. Advantageously, this removes some of the aluminum present in the coating that could contaminate the weld seam and degrade its mechanical properties.

[0056] In certain embodiments, a blank 26 used to manufacture a rear underbody structure 2 comprises at least one subblank having at least one side having an emissivity-enhancing upper layer. The emissivity-enhancing upper layer is applied to the outermost surface of the subblank. The emissivity-enhancing upper layer allows the surface of the subblank to have a higher emissivity compared to the same subblank not coated with the emissivity-enhancing upper layer. The emissivity-enhancing upper layer can be applied to either the upper or lower side of the subblank. The emissivity-enhancing upper layer can also be applied to both sides of the subblank.

[0057] If the subblank has the aforementioned metal coating, the emissivity-enhancing upper layer is applied to the top of the metal coating. In fact, for the emissivity-enhancing upper layer to increase the surface emissivity, it needs to cover the outermost surface of the subblank.

[0058] Advantageously, the emissivity-increasing upper layer allows for an increase in the heating rate of the subblank, and therefore increases the productivity of the heating step in the hot stamping process.

[0059] In certain embodiments, the blank 26 consists of n subblanks S1, S2, ... Sn sorted in increasing order of thickness. The emissivity-increasing top layer is applied to at least one face of the subblank having the maximum thickness. In certain embodiments, the emissivity-increasing top layer is applied to at least one face of the subblank having the maximum thickness and the subblank having a thickness just below the maximum thickness. In certain embodiments, the emissivity-increasing top layer is applied to x subblanks having the maximum thickness, where x is an integer greater than or equal to 1. Advantageously, by applying the emissivity-increasing top layer to the thicker set of subblanks, it is possible to achieve a more uniform heating rate during the heating step of the hot stamping process between the thicker and thinner subblanks. In fact, thinner subblanks naturally heat up faster than thicker subblanks because they are thinner and require less energy to reach the same temperature. By targeting the thicker subblanks with the emissivity-increasing top layer, the difference in heating rates between blanks of different thicknesses can be reduced, and thus a more uniform heating rate can be achieved between the thicker and thinner subblanks. Furthermore, by targeting thicker subblanks with an emissivity-enhancing top layer, it is possible to increase the size of the process window in the heating step of the hot stamping process for the blank. When hot stamping large parts with large thickness differences, one concern is the large differences in the process windows (including heating time and heating temperature in particular, among other parameters) required to achieve the desired microstructure and coating properties of different subblanks. The process window required to achieve the desired properties across the entire blank is the intersection between each process window of the individual subblanks. By applying an emissivity-enhancing top layer onto thicker subblanks, the process windows of the individual subblanks can be brought closer together, thereby increasing the size of the intersection between the process windows of all subblanks, i.e., increasing the overall process window of the tailored blank.

[0060] In certain embodiments, the emissivity-enhancing upper layer has a thickness of 2 to 30 microns. In certain embodiments, it is composed of a polymer which does not contain silicon, contains more than 1% by weight of nitrogen, and contains 3 to 30% by weight of carbon pigment.

[0061] In certain embodiments, the blank 26 used to manufacture the rear underbody structure 2 further comprises at least one metal patch 31, as shown in Figure 8, to locally increase the strength of the part. In certain embodiments, the patch 31 is attached by spot welding. In certain embodiments, the patch 31 is attached by laser welding. The patch 31 is designed to be attached, for example, to the chassis structure, or to areas that need to be reinforced due to mechanical problems such as folding of parts detected during crash testing.

[0062] Generally speaking, Patch 31 provides highly localized reinforcement across larger parts, thus offering the advantage of further optimizing the overall strength and thickness distribution of the tailored blank and keeping the overall weight and cost of the part low.

[0063] Patch 31 is made, for example, from press-hardened steel. Patch 31 is coated, for example, with an aluminum-based metal coating.

[0064] In certain embodiments, the patch 31 is coated with an emissivity-increasing upper layer in order to increase the heating rate and thus reduce the difference in heating rates in the region of the patch 31 related to the excess thickness of the patch 31, as described above.

[0065] In certain embodiments, the patch is applied to an area including a portion of the weld seam 25. As shown in Figure 8, we refer to this as a weld seam reinforcement patch 32. Such a patch 32 has exactly the same characteristics as the features described above and optional features. Such a patch 32 reinforces the weld seam 25. The weld seam 25 is the area where a discontinuity exists between the two subblanks, and this area results in localized inertial fluctuations, which can lead to plastic hinge-type collapse when subjected to high loads generated by the collision. Furthermore, due to the heat input of the welding process, the weld seam is surrounded by a heat-affected zone, and the weld seam itself has a specific chemical composition resulting from the melting and mixing of the subblanks, their metal coatings if present, and the filler wire used for welding if present. This complex thermal, chemical, and metallurgical situation, combined with the discontinuity problem described above, means that there may be variations in strength around the weld 25, and that the weld may fracture further in areas of the weld 25 when subjected to excessive load. Such plastic hinge phenomena can be prevented by reinforcing the weld seam 25 with a weld seam reinforcement patch 32. The weld seam reinforcement patch 32 is attached, for example, by welding it to the blank 26. In certain embodiments, the attachment point between the weld seam reinforcement patch 32 and the blank 26 is not located in the weld seam 25 region so as not to interfere with the mechanical properties of the weld seam 25.

[0066] When using press-hardened steel coated with an aluminum-based metal coating on a blank 26, the hot stamping process induces the formation of an interdiffusion layer between the steel and the metal coating on the hot-formed part. The interdiffusion layer is a result of high-temperature interdiffusion between Fe coming from the steel and moving toward the metal coating, and Al coming from the coating and moving toward the steel. The thickness of the interdiffusion layer has been shown to correlate with further in-use properties of the part, such as the ability of the part to be successfully assembled to the rest of the body by spot welding. In particular, hot-formed parts with an interdiffusion layer thickness between 3 and 15 microns have been shown to have good in-use properties. More preferably, hot-formed parts with an interdiffusion layer thickness between 3 and 10 microns have been shown to have excellent in-use properties.

[0067] In certain embodiments, the thickness of the interdiffusion layer within the aluminum-based metal coating area of ​​the rear underbody structure 2 is between 3 microns and 15 microns. In certain embodiments, the thickness of the interdiffusion layer within the aluminum-based metal coating area of ​​the rear underbody structure 2 is between 3 microns and 10 microns.

[0068] The fact that the rear underbody structure 2 is manufactured as a single piece integrating the side members 4 and at least one cross member 5 means that the shape of the side members 4 must be adapted in the cross member mounting zone 20. In particular, the shape of the inner wall 42 must be adapted as described above. The presence of a continuous inner wall 42 is not possible in the mounting zone 20, as is the case with the latest multi-part solutions.

[0069] In the case of a rearward impact that applies a compressive load to the side member 4, the inventors have found that it may be advantageous to at least partially retain the inner range 45 within the cross member mounting zone 20. In fact, although we do not wish to be bound by theory, it appears that the compressive force is transmitted along the side member 4, mainly through the inner range 45 and outer range 46 of the side member. If there is a large discontinuity in the inner range 45 across the cross member mounting zone 20, the compressive force generated by the impact will not be properly transmitted to the rest of the vehicle structure, resulting in stress concentration in the cross member mounting zone 20. This can lead to serious vulnerability of the component in this area and inadequate vehicle safety performance, for example, to serious buckling in the mounting zone 20.

[0070] In certain embodiments, the inner range 45 extends over at least a portion of the cross member mounting zone 20, as is clearly visible in the perspective view of Figure 6. For example, as shown in Figure 6, this can be achieved by introducing a step between the horizontal wall 41 of the side member 4 and the horizontal wall 51 of the cross member 5 in the cross member mounting zone 20. In this step configuration, the cross member range 55 follows the inner range 45, forming a step. Although not shown in the accompanying figures, another possibility for providing an extension of the inner range 45 over at least a portion of the transition zone 20 is to introduce a bead in the transition zone 20 instead of the step described above. The bead can be formed, for example, by a shape similar to the step described above, followed by a symmetrical step according to a plane of symmetry perpendicular to the orientation of the cross member 5. This type of bead configuration allows for maintaining the same height for the horizontal walls 41 and 51 while maintaining the presence of the inner range 45 over at least a portion of the transition zone 20.

[0071] In certain embodiments, the rear underbody structure 2 comprises a geometric modification 22, as shown, for example, in Figure 7. The geometric modification 22 allows stress resulting from an impact to be concentrated in a specific area of ​​the part in order to protect other, more vulnerable areas of the part. For example, to protect the weld seam 25, the geometric modification can be positioned near the weld seam 25 (represented by the dashed line in Figure 7) that connects different sub-blanks of the tailored blank used to manufacture the rear underbody structure 2. With the aforementioned sensitivity of the weld seam 25 under high loads in mind, the use of the geometric modification 22 acting as a trigger in the event of a collision can favorably protect the weld seam 25 and ensure, for example, that the formation of a plastic hinge occurs away from the weld seam 25, and thus prevent crack formation in the part.

[0072] As described above, the rear underbody structure 2 according to the present invention comprises at least one cross member 5, but may also comprise several cross members 5. It may comprise, for example, two, three or more cross members 5. Figure 4 is a schematic top view of the rear underbody structure 2 comprising two cross members 5. Integrating multiple cross members 5 within the same rear underbody structure 2 means increased cost and manufacturing simplicity benefits. This also means that the blank 26 used to manufacture the rear underbody structure 2 has better rigidity thanks to the presence of several cross members 5 connecting the side members 4. This may be particularly advantageous in handling the blank 26 during the manufacturing process and may be even more advantageous in the case of hot stamping, which involves handling a blank with lower mechanical strength during the process of transferring the blank from the austenitizing furnace to the hot stamping press, where the blank is heated at high temperatures.

[0073] The present invention also relates to a vehicle 1 having the rear underbody structure 2 described above.

[0074] The present invention also relates to a process for manufacturing the rear underbody structure 2 described above and assembling it to the rest of the vehicle body.

[0075] This method is as follows: - Steps to provide a tailored blank 26, -For example, the step of stamping a tailored blank 26 by hot stamping, -Includes a step for attaching the rear underfloor structure 2 to the body of the vehicle 1.

Claims

1. A rear underbody structure (2) for an automobile (1), comprising first and second side members (4) and at least one cross member (5) connecting the first and second side members (4), wherein the rear underbody structure (2) is made by stamping a single tailored blank (26) having at least two subblanks.

2. The rear underbody structure (2) according to claim 1, wherein the rear underbody structure (2) is made by hot stamping.

3. The rear underbody structure (2) according to claim 1 or 2, wherein the rear underbody structure (2) comprises at least two cross members (5).

4. Each side member (4) is: A horizontal wall (41) and an inner wall (42) connected by an inner range (45), It comprises at least one crossmember mounting zone (20) corresponding to the portion of the side member (4) to which the corresponding crossmember (5) is attached, The rear underbody structure (2) according to any one of claims 1 to 3, wherein the inner range portion (45) extends along at least a portion of the cross member mounting zone (20).

5. The rear underbody structure (2) according to any one of claims 1 to 4, wherein the tailored blank (26) comprises at least one subblank coated with an aluminum-based metal coating.

6. Rear underbody structure (2) according to any one of claims 1 to 5, wherein the tailored blank (26) comprises at least one subblank coated with an aluminum-based metal coating, the subblank containing 2.0 to 24.0% by weight of zinc, 1.1 to 12.0% by weight of silicon, optionally 0 to 8.0% by weight of magnesium, and optionally additional elements selected from Pb, Ni, Zr, or Hf, the content of each additional element being less than 0.3% by weight, with the remainder being aluminum and optionally unavoidable impurities.

7. The rear underbody structure (2) according to any one of claims 1 to 6, wherein the tailored blank (26) comprises at least one sub-blank having an emissivity-increasing upper layer on at least one side.

8. The rear underbody structure (2) according to any one of claims 1 to 7, wherein the tailored blank (26) comprises at least one subblank made of press-hardened steel having an ultimate tensile strength after hot stamping of more than 1800 MPa.

9. Rear underbody structure (2) according to any one of claims 1 to 8, wherein the tailored blank (26) comprises at least one subblank, the subblank being made of press-hardened steel having a hot-formed yield strength between 700 and 950 MPa, a hot-formed ultimate tensile strength between 950 and 1200 MPa, and a hot-formed bending angle greater than 75°.

10. The rear underbody structure (2) according to any one of claims 1 to 9, wherein the tailored blank (26) comprises at least one metal patch (31).

11. The rear underbody structure (2) according to claim 10, wherein at least one metal patch (31) comprises an emissivity-increasing upper layer.

12. The rear underbody structure (2) according to any one of claims 1 to 11, wherein the tailored blank (26) comprises at least one weld seam reinforcement patch (32), the weld seam reinforcement patch (32) being attached to an area comprising a weld seam (25).

13. The rear underbody structure (2) according to claim 12, wherein at least one weld seam reinforcement patch (32) includes an emissivity increasing upper layer.

14. The rear underbody structure (2) according to any one of claims 5 to 13, wherein the thickness of the interdiffusion layer within the aluminum-based metal coating area of ​​the rear underbody structure (2) is between 3 microns and 15 microns.

15. The rear underbody structure (2) according to any one of claims 1 to 14, wherein the thickness of the interdiffusion layer within the aluminum-based metal coating area of ​​the rear underbody structure (2) is between 3 microns and 10 microns.

16. An automobile (1) comprising a rear underbody structure (2) according to any one of claims 1 to 15.

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